So how do you get people motivated to sync down to i70 or whatever and go challenge old content?

By gating rewards behind it, duh. It doesn't necessarily have to be loot, just titles, mounts and minions would be fine. At a stretch, you could offer extra tomestones as you decrease the level in the level sync. This way, newer statics can challenge the old content in the way that it was meant to be, while old statics can dust off their old experience and return to that content for some new rewards.